Room-temperature oxidation of hypostoichiometric uranium-plutonium mixed oxides U1-yPuyO2-x – a depth-selective approach

The oxygen content of a nonstoichiometric oxide compound is, in numerous cases, decisive regarding the material properties (electrical and/or thermal conductivity, melting point, etc). A phase separation is known to occur in oxygenhypostoichiometric uranium-plutonium mixed oxides beyond 20 mol% Pu. During its fabrication, the (U1-yPuy)O2-x mixed oxide can be subjected to several atmospheres, at room temperature, before being enclosed in the final fuel rods (e.g. storage after reducing sintering). Using a multi-scale approach (from the sample’s bulk to its surface), we reveal the evidence of spontaneous room-temperature oxidation of biphasic (U1-yPuy)O2-x compounds, with y = 0.28 and 0.45, even when exposed to low moisture and oxygen levels. This oxidation is, in both cases, significant within very short timescales. We believe our results are of utmost importance in the prospect of characterizing oxygen-hypostoichiometric uranium-plutonium mixed oxides with high Pu content.